I'm used to what's called local loop unbundling in the UK and Europe - which is the removal of the last mile monopoly, and all ISPs simply putting a DSLAM in exchanges. Initially this was localised, and there were smaller local players, but they're now effectively all nationwide.
DSL (with a single provider) is my only option for wired internet at home. Satellite and cellular are both available, but with higher prices and sever data caps.
I have an office 1.5 miles away that also has a cable line that can do ~120Mbps down and ~5 up, but the cable lines end about a mile from my house.
It did exist in some form after the 1996 Telecommunications Act so it happened somewhat with DSL providers, but the laws were narrow, didn't address other issues that arise as a result of the law, and didn't include cable/fiber/etc
So any speed differences between ISPs are mostly down to the quality or not of the ISP's network. Which means roughly Talktalk bad, A&A good, but the sync speed should be the same. :)
You have ISP #1, who have relatively modern infrastructure and bandwidth.
You may or may not have ISP #2, who have antiquated infrastructure they don't maintain that delivers bandwidth circa 2005.
You probably also have ISP #3, who use alternate delivery technologies (wimax, satellite, etc) to deliver low bandwidth at exorbitant prices.
So if Comcast decides not to sell me service, I'm screwed. They could charge as much as they want and to a point I'd just have to suck it up and pay. I assume the reason it isn't multiple times more expensive is because there is some regulation from the PUC capping what they can charge.
Some have more. Some have less. I've never lived somewhere that had only one choice though, so I'm not sure how common it is.